1. Field of the Invention
The present invention relates to a superconducting current measuring circuit and a superconducting current measuring device using a superconductor and suited for measuring a current waveform with high time accuracy.
2. Description of the Related Art
There have been conventionally known some techniques for providing means for measuring the waveform of a current flowing through wiring. FIG. 1 is a typical view showing a conventional current measuring method. If a value Z of impedance of a wiring 51 is known, a voltage V between a first end 53 and a second end 54 of the wiring 51 is measured by a semiconductor sampler 52, which can measure a voltage with time accuracy of several pico-seconds. Using the measured voltage V, a current I is obtained according to a formula of I=V/Z. This is an ordinary method, which method will be referred to as "the first prior art" hereinafter.
There is also known a method of measuring a current waveform by means of a magnetic sensor using electromagnetic inductance, as described in the Transactions of The Institute of Electrical Engineers of Japan, Vol.117-A, No.5, May, 1997, pages 523 to 530. This method will be referred to as "the second prior art" hereinafter. FIG. 2 is a typical view showing a measuring method according to the second prior art. In this method, the output voltage of a magnetic sensor 55 using electromagnetic induction is measured by a spectrum analyzer 56. Next, the waveform of a current flowing through a wiring 14 on a measurement target 13 is obtained as the deconvolution of a sensor factor which is a ratio of a received magnetic field with each frequency to an output voltage.
Furthermore, there is proposed a measuring method using a superconducting loop (Japanese Patent Unexamined Application Publication No. 7-135099). This method will be referred to as "the third prior art" hereinafter. FIG. 3 is a typical view showing a measuring method according to the third prior art. In this method, a magnetic field generated due to an ion beam (flow of charged particles=current) is detected by a superconducting loop (DC-SQUID) 59 including Josephson junctions 57 and 58 and an ion beam current is measured. In the third prior art, the voltage between both ends of the superconducting loop 59 biased with a current source 60 so as to provide a constant voltage state is measured by a voltmeter 61. The displacement of the voltage generated when a magnetic field is applied in this state from the initial voltage value is fed back to the current source 60 and bias current is adjusted to thereby make voltage constant. The magnitude of the applied magnetic field is measured from the variation of the bias current at this time and an ion beam current value is obtained.
A superconducting sampler is described in the IEICE TRANSACTIONS ON ELECTRONICS, VOL. E80-C, No. 10, October, 1997, pages 1226 to 1232. FIG. 4A is a circuit diagram showing the superconducting sampler described therein. In addition, the Extended Abstract of the 45th Applied Physics Association Meeting of Japan, meeting No. 28a-Z-11, March, 1998, page I-225 describes that if the superconducting sampler mentioned in the IEICE TRANSACTIONS ON ELECTRONICS, VOL. E80-C, No. 10, October, 1997, pages 1226 to 1232 is used, it is possible to measure current waveforms with time accuracy of pico-seconds. FIG. 4B is a timing chart showing the operation of the superconducting sampler shown in FIG. 4A.
First, a feedback current I.sub.f is supplied to the first input terminal 171, thereafter, a measurement target current I.sub.S is inputted. If a trigger current I.sub.tr is supplied from the second input terminal 172 at certain timing, a first Josephson junction 173 is switched on. As a result, an SFQ (Single Flux Quantum) enters a first superconducting loop including the first Josephson junction 173, a second Josephson junction 174, a third Josephson junction 175 and a first inductance 178. Following this, a first circulating superconducting current flows through the first superconducting loop.
In addition, an SFQ opposite in direction to the above SFQ enters a second superconducting loop including the first Josephson junction 173 and a second inductance 179. If the critical current value of the second Josephson junction 174 is set lower than the first circulating superconducting current, the current flowing through the second Josephson junction 174 falls while rising by the switching of the second Josephson junction 174. As a result, a pulse current I.sub.P occurs and flows into the third Josephson junction 175.
The third Josephson junction 175 is referred to as a comparator junction. Since the feedback current I.sub.f and the measurement target current I.sub.S already flow through this junction, the feedback current I.sub.f, the measurement target current I.sub.s and the pulse current I.sub.P are added up to one another. If the sum of the three currents is equal to or higher than the critical current of the third Josephson junction 175, the third Josephson junction 175 is switched on. As a result, a third circulating superconducting current flows through a third superconducting loop including the third Josephson junction 175, a third inductance 180 and a coupling inductance 181 coupled with a readout SQUID (Superconducting quantum interference device). The third circulating superconducting current causes the generation of voltage between both ends of the readout SQUID including the fourth Josephson junction 176 and the fifth Josephson junction 177. If the sum of the three currents is less than the critical current value of the third Josephson junction 175, the junction 175 is not switched on and no voltage is generated between both ends of the readout SQUID.
If negative currents are carried as the second circulating superconducting current and the third circulating superconducting current at the end of each measurement cycle, the first Josephson junction 173 and the third Josephson junction 175 are switched and the superconducting sampler is reset.
The above operation is repeatedly conducted while changing the value of the feedback current I.sub.f, and the lowest feedback current I.sub.f with which an output voltage occurs is obtained. The obtained value is compared with the value of the lowest feedback current I.sub.f with which an output voltage occurs while the measurement target current I.sub.S is 0, thereby obtaining the value of the measurement target current I.sub.S at timing at which the pulse current I.sub.P occurs. Next, the timing for supplying a trigger current I.sub.tr is changed, timing at which a pulse current I.sub.P occurs is shifted and the same measurement operation is repeated. In this way, the waveform of the measurement target current I.sub.S can be measured using the superconducting sampler shown in FIG. 4A. The above-stated measuring method will be referred to as "the fourth prior art" hereinafter.
The first prior art shown in FIG. 1 is, however, applicable only to a case where the impedance of wiring is known. Normally, there are many cases where the impedance of wiring is not known. As for, for example, in regard to wiring of a semiconductor large scale integrated circuit (LSI), since contact holes exist and wiring structure is complicated, the inductance of the wiring is normally unknown. In this case, a current waveform cannot be measured using the semiconductor sampler.
The second prior art shown in FIG. 2 has disadvantages in that detection sensitivity for low frequency components is low and waveform reproducibility is not satisfactory since electromagnetic induction is employed. Besides, the measurable upper limit frequency which is determined with an L/R time constant where R is the resistance of a sensor and L is an inductance, is limited to as low as 1 GHz.
The third prior art shown in FIG. 3 has disadvantages in that it takes long time to feed back a bias current for keeping the voltage of the SQUID constant and measurement time accuracy is considerably low.
The fourth prior art shown in FIGS. 4A and 4B has an advantage in that a current waveform can be measured with high time accuracy. However, the fourth prior art requires directly supplying a measurement target current to the sampler through wiring, so that non-contact measurement cannot be made. For this reason, the fourth prior art has a disadvantage in that a measurement target current may be greatly influenced depending on measurement type. On the other hand, there are some cases where a current is not necessarily measured in a non-contact manner such as a case where a measurement target current flows through the ground. Nevertheless, due to the fact that the superconducting sampler is arranged under low temperature environment when the sampler is used, it is necessary to introduce the measurement target current from a measurement target at room temperature to this low temperature environment using a long signal line. As a result, if frequency becomes higher, problems, such as the distortion of a current waveform and crosstalk, occurs.